Field of the Invention
The present invention relates generally to indicating devices and, more particularly, to a device for indicating when the torque on a shaft exceeds a predetermined limit.
Description of the Prior Art
Many types of valve actuators include an actuator shaft which is rotatably motor driven in one direction to open the valve and in the opposite direction to close the valve. Furthermore, the actuator shaft is subjected to relatively high torque loads when the valve reaches or is initially moved from either its open or closed position. In the event of valve malfunction, however, the actuator shaft can be subjected to excessive torque loads which can damage the valve components including the valve motor.
There have been a number of previously known devices designed to measure the torque on the actuator shaft to deactivate the valve motor when the torque exceeds a predetermined limit. These previously known devices, however, have not proven wholly satisfactory in use.
One disadvantage of these previously known devices is that they are complex and unreliable in operation. Consequently, these previously known torque limiting devices frequently fail to deactivate the valve motor despite a torque overload thus damaging the valve components.
A still further disadvantage of many of the previously known devices for deactivating the actuator motor at a predetermined torque overload is that the torque limit at which the valve motor is deactivated is not adjustable by the ultimate user but rather is preset at the place of manufacture. In addition, it is conventional to set the torque overload limits just below the maximum permissible limits. Many valves, however, slowly deteriorate during extended operation and the maximum torque applied to the valve actuator shaft also gradually increases with the deterioration of the valve. Finally, the maximum permissible torque is exceeded and at this time, the valve essentially is rendered inoperable. Immediate repair or replacement of the valve is then required before the operation of the fluid system can continue. In many circumstances, however, the immediate replacement or repair of the valve is either impractical or impossible thus resulting in prolonged downtime of the fluid system.